This invention pertains to film balances of the type used to measure the surface tension or surface energy of a thin film. More particularly, this invention pertains to such balances of the Langmuir or horizontal type, as distinguished from balances of the Wilhelmy or vertical type. In the Langmuir balance, an elongated horizontal trough is filled with a liquid substrate, usually water. A barrier spanning the width of the trough separates the upper surface of the substrate into two portions. The thin film to be investigated, usually an oil or the like, is formed on one side of the barrier by depositing a small amount of film material, or a mixture of film material and volatile solvent, on the surface of the substrate. The film material then spreads out from the point of deposit, forming a monolayer thin film on the surface of the substrate. The surface area of the coated substrate portion is then reduced, reducing the area of the thin film to be studied, and compressing the thin film against the aforementioned barrier. A force is, in this manner, exerted upon the side of the barrier contacting the thin film. Inasmuch as the opposing side of the barrier bounds a surface of uncoated substrate, a surface energy (surface tension) differential is set up on the barrier. Prior art barriers were typically very thin, comprising paraffin-coated paper and the like, and floated on the clean uncoated surface, so as to be capable of movement under the force of the thin film which was compressed against one side of the barrier. The movement of the barrier, more commonly termed a float, upon the horizontal substrate surface was detected by knife-edge or torsion wire movements which were attached to the float by vertical members extending normal to the substrate surface. However, difficulties were experienced with both types of movements. The earliest film balance in use today was reported by Langmuir in 1921. In that arrangement, the float was mounted from above for pivotal deflection, by a knife-edge movement. One side of the knife-edge movement was attached through a transverse horizontally extending arm to a weight pan, to form one-half of a scale, the weight in the pan balancing the pivotal deflection of the knife edge, caused by the differential surface energy applied to the float. Such arrangements were quickly discarded in favor of the second, torsion wire type, when the instability of the knife edge proved to detrimental to more accurate measurements. The knife edge movement proved unsatisfactory in that the forces applied to the knife edge which caused it to rock, could if large enough, cause the knife edge to slide horizontally, destroying the zero set and measuring ability of the device. The knife edge movement presented further problems in that, if tilted too far from a balanced vertical position, would become unable to support itself, causing the knife edge to fall to one side, again destroying the zero set and measuring ability of the mechanism. Knife-edge movements also display a dead region adjacent a vertical or balanced orientation, thereby defining a minimum deflection limit of the useful operating range of such a balancing mechanism.
The second type of balance, currently in use today suspends the float from torsion wire movement, wherein the horizontal deflection of the float is transformed into a torsion force which rotatably deflects the torsion wire from which it is suspended. While overcoming the instability problems of the knife-edge mechanism, this arrangement suffers from its own peculiar drawbacks. In order to provide the high sensitivity currently required of such balances, the torsion wire must be made as lightweight as possible. However, as the guage of the torsion wire is reduced, its ability to support massive float mechanisms is also reduced. Accordingly, only very light float arrangements can be used with this the torsion-type measurement device.
Further difficulties were encountered in prior art float constructions which were known to swamp at high differential surface tensions, thus permitting thin film material to leak past their end portions, thereby destroying measurement accuracy.
It is therefore an object of the present invention to provide a thin film balance having improved sensitivity while providing the support necessary to suspend more massive floats as well as associated measuring equipment.
It is a further object of the present invention to provide a highly sensitive film balance having a wide range measuring capability. The film balance having such wide range capability must be conveniently operable over its entire operating range, without requiring breakdown or set-up of various portions of the film balance mechanism.
Another object of the present invention is to provide an improved float for a film balance, which is resistant or virtually unaffected by swamping at high differential forces. Such float must also present an impassable barrier to the thin film under examination, i.e., thin film material must not be allowed to leak past the ends of the float.
It is yet another object of the present invention to provide a film balance having the advantages set forth above, which further provides a highly accurate automated balancing means.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.